Baluns are well-known passive electrical devices. The term “balun” is derived from the abbreviation of the two terms ‘balanced’ and ‘unbalanced’. Baluns are 3-port devices which convert signals from an unbalanced transmission line to a balanced transmission line and vice-versa. The two balanced ports should provide a signal equal in amplitude with a 180 degree phase difference.
Microwave balun devices can be implemented in various ways, such as in transformer-type arrangements, coupled transmission lines and transmission line junctions. It is known from US2005/0105637 and Bialkowski and Abbosh (M E Bialkowski and A M Abbosh, IEEE Microwave and Wireless Components Letters, Vol. 17, No. 4, April 2007) how to implement baluns using microwave techniques involving microstrips and slotlines. However, it would be desirable to improve the performance characteristics of these devices. In particular, it would be desirable to achieve a wider frequency range over which useful operation of the device can be achieved.